Goal: Human visual performance varies across the visual field. Performance decreases with eccentricity in a variety of tasks. Performance can also differ at isoeccentric locations. Both contrast sensitivity and spatial frequency discrimination are better along the horizontal meridian than the vertical meridian resulting in a horizontal-vertical anisotropy (HVA) and at the lower than the upper region of the vertical meridian, resulting in a vertical meridian asymmetry (VMA). Furthermore, the limit of spatial resolution - the spatial frequency at which discrimination performance is at chance (cutoff point) - is lower in the periphery than in the fovea. Here, we investigated whether the HVA and the VMA extend to spatial resolution: Do the cutoff points in resolution vary at isoeccentric locations in the visual field?

Method: Observers performed an orientation discrimination task on Gabors of varying spatial frequency at four isoeccentric locations of the visual field (North, South, East and West). In each trial, a tilted Gabor patch was presented in one of the above-mentioned locations at either a parafoveal or peripheral eccentricity (4 vs. 8 degrees). We obtained psychometric functions for orientation discrimination with respect to spatial frequency for each location and then estimated the spatial frequency cutoff point.

Results and conclusions: Cutoff spatial frequencies were significantly lower, indicating poorer performance, along the vertical meridian (North and South) than the horizontal meridian (East and West) at both parafoveal and peripheral eccentricities. In addition, they were lower for the North compared to the South location. These results show that the limit of spatial resolution, assessed by spatial frequency cutoff, differs at isoeccentric locations in the visual field. We propose that the observed differences originate from underlying anatomical inhomogeneities (e.g., cone density and cortical extent of the field representation) along the meridians.